The object of the present invention is a tire made from several mixes comprising as the principal charge a non-reinforcing charge such as silica, or mixes with low carbon black contents, at least one of these mixes forming the tread. More particularly, it concerns a tire whose internal temperature may increase not inconsiderably when rolling, such as a tire designed, for example, to support heavy loads. It also concerns a process designed to produce such a tire and equipment for carrying out such a process.
Since environmental problems are becoming increasingly crucial and fuel economy and the suppression of pollution produced by motor vehicles have become a priority, one of the tire manufacturers"" objectives is to produce a tire which combines the properties of very low rolling resistance, excellent adhesion to both dry and wet or snow-covered or icy ground, very good wear resistance, and finally low rolling noise.
To achieve that objective, a tire was proposed in European Patent EP A 501 227, which has a tread comprising silica as its main reinforcing charge. Though this solution provides the best compromise between the aforementioned group of very contradictory properties, it has been found that during the operation of the vehicle, tires with a tread whose main reinforcing charge is silica have the disadvantage that they accumulate more or less high levels of static electricity, which is produced by friction between the tire and the road when the vehicle is rolling, because silica is an electrically non-conductive material.
When certain particular conditions co-exist, the static electricity so accumulated in a tire can give rise to a disagreeable shock to the occupant of a vehicle when the occupant touches the vehicle body. It can also hasten the ageing of the tire because of the ozone generated by the electrical discharge. Depending on the nature of the ground and the vehicle, it can in addition cause the radio fitted in the vehicle to function poorly because of the interferences it produces.
This problem of static electricity accumulation in a tire and most of the disadvantages associated therewith is a very old one and already existed when the reinforcing charge used was carbon black.
Patent application EP 0 658 452 A1 describes the adaptation of principles known for a long time, to a so-termed modem tire. This adaptation provides a solution of the main problems associated with the solutions proposed in various older documents, and in particular the harmful heterogeneities introduced in tire structures. The solution proposed is to insert a strip of conductive rubber mix or xe2x80x98insertxe2x80x99, preferably extending all around the circumference of the tire and connecting the surface of the tread either to one of the crown plies, or to the carcass reinforcement, or to any other part of the tire that conducts electricity sufficiently well, the necessary electrical conductivity being conferred by the presence of a suitable form of carbon black.
Though such a solution is perfectly viable for a tire having a tread which consists of just one and the same non-conductive mix, for example the tread of a touring vehicle, this does not apply in the case of a tire comprising several layers of rubber mixes above the crown reinforcement and the carcass reinforcement, as is the case with any tire that can roll with a stable and high operating temperature, such as those fitted to heavy or high-speed vehicles.
In fact, if for any reason it is wished to produce such a tire with a layer or internal portion of the tread (the portion not in contact with the ground), which is non-conductive, between the crown reinforcement and the outer portion of the tread (the portion in contact with the ground) which has been made conductive by the presence of a circumferential insert or striation, the said internal portion must be made conductive. Similarly, a layer between the carcass reinforcement and the crown reinforcement, one which has the familiar excess thickness in the area of the edges of the crown plies, must also be made conductive if it is not already so. A first solution for obtaining a conductive tread is to co-extrude the inside and outside portions of the tread and to provide the combination so formed with a conductive circumferential insert. This solution is unsatisfactory for several reasons, two of which are worth mentioning: in the type of tire considered, the total thickness of the tread is too large; besides, it may be advantageous for the conductive inserts, respectively of the inside and outside layers of the tread not to be made using the same quality of rubber mix.
As described in French Patent Application FR 97/02276, another solution is to provide electrical connection between two conductive layers, or layers rendered conductive, separated by a non-conductive layer, by means of at least one strip of rubber mix with small thickness, width and length, positioned between the two weld faces of the non-conductive layers and in contact with the means used to make the two layers joined by the connection conductive. Though industrially satisfactory, this method entails positioning a supplementary product and therefore involves additional manufacturing costs.
A third solution consists in providing each non-conductive portion with a circumferential insert having a rectilinear circular path, or circular striation, after extrusion of the said portion by the usual extrusion methods, and then joining the two products together before they are positioned on the crown reinforcement. In that the striations are generally very thin so as not to affect the physical properties of the compositions constituting the two portions of the tread, the thickness of these inserts in the tire viewed in cross-section being of the order of 0.01 to 2.0 mm, such a solution entails that the path of the striation in the outside portion of the tread on the contact surface between the two portions should be perfectly aligned with or centered on the path on the said contact surface of the striation in the inside portion of the same tread.
However, the mechanical properties of rubber compositions before vulcanization are very poor, since the raw rubber mix may be in the form of a very soft paste or conversely a very hard agglomerate. Whatever the means used to work such mixtures, it is difficult to achieve perfect control of the geometry of the semi-finished product consisting of the combination of the two unvulcanized portions of the tread: coincidence or alignment between the respective paths of the two striations on the contact surface between the two portions is very difficult to achieve, so the solution involving two circular striations cannot be considered structurally optimized and industrially viable, from the standpoint either of cost or of the performance obtained. The manufacture of a tire designed in this way would be more akin to precision engineering than to an industrial process.
One of the invention""s aims is to dissipate electrostatic charges induced by the rolling of a tire comprising several electrically non-conductive mixes without significantly affecting the properties of the tire. Another aim is to obtain a tire as simple and cheap as possible to produce in terms of the material costs and/or manufacturing costs involved.
The present invention proposes a tire comprising at least two radially adjacent layers of electrically non-conductive rubber mixes the said two layers sharing a common contact surface, characterized in that each layer contains a circumferential insert of electrically conductive mix whose footprint on the said contact interface is circumferential, the insert of the first of these layers having on the said contact surface a circumferential footprint of circular path of width e, the insert of the second layer having on the contact surface a circumferential footprint with a path of width exe2x80x2 which crosses and presents crests on either side of the circular path of the first layer insert, such that around the circumference there are numerous points of contact between the two paths which ensure electrical connection between the two conductive elements, and the second layer insert has a maximum crest-to-crest width at least equal to 10 mm.
The path of the circumferential footprint of the second layer insert which is the most simple to produce industrially, is an undulating path and more particularly a periodic path. A path is termed periodic when it can be represented by a periodic function and could be, for example, an undulating path, a zigzag path, a path consisting of triangles, a sinusoidal path, or a path consisting of rectangles or trapezoids. Thus, the said path can be defined by an amplitude and a wavelength, by analogy with periodic motions. The amplitude a may very along the circumference of the contact surface between the layers, but is preferably constant, and the median axes of the two paths on the contact surface will be parallel to and separated from the equatorial plane of the tire by distances whose difference is at most equal to half the amplitude a/2 of the said periodic path.
It is advantageous because most simple, for each layer to contain an insert which, when viewed in cross-section, has a rectangular area over the full thickness of the layer.
In by far the majority of cases, the two non-conductive layers will be the inside and outside portions of the tread and electrical connection must be ensured between the ground and the crown reinforcement consisting of metallic cables set in a rubber mix that has been made conductive by the presence of carbon black in its composition. The non-conductive layers may also be on the one hand a profile of triangular cross-section that separates the carcass reinforcement from an edge of the crown reinforcement, and on the other hand the inside and outside portions of the tread. Connection must then be ensured between all three layers.
In the case of a tread, it is best for the portion of the said tread having an insert with a circumferential footprint of circular path on the contact surface between the two portions, to be the outer part of the tread, the other portion, preferably the inner part of the tread, being provided with a striation whose circumferential surface has a periodic path, preferably undulating.
A layer, whether it is part of the tread or an intermediate section between the carcass reinforcement and the crown reinforcement, is generally obtained in the non-vulcanized state by extrusion using an extruder. The process designed to obtain the combination of the two layers which are to be conductive, for example the portions of a tire tread, consists in the separate extrusion of each layer, for example each portion of the tread, inserting in each layer a striation, which is circumferential and rectilinear in one layer, for example the outer portion of the tread, but which is periodic in the other layer, or example the inner portion of the said tread, and then joining the two layers by the usual methods, for example the two said portions, to form the unvulcanized tread. The process according to the invention for obtaining the layer with a periodic striation consists in the following:
extruding the said layer using an extruder comprising an extrusion head provided with a flow channel which opens into an extrusion orifice,
cutting the section produced, positioned on a means of conveyance located downstream from the extrusion orifice, with the aid of a cutting device,
extruding the striation of conductive rubber mix in the unvulcanized condition and inserting it between the two walls of the cut by means of the nozzle of a mobile micro-extruder, the said nozzle being directed against the wall of the said means of conveyance and undergoing a periodic movement parallel to the said wall and of amplitude at least equal to 10 mm.
Advantageously, the process according to the invention uses an extruder familiarly known as a roller die extruder, in which the profile or section of the product extruded is defined on the one hand by the surface of the roller onto which the rubber mix is extruded and on the other hand by the fixed wall of an extrusion blade which cooperates with the said surface to delimit an extrusion orifice. The process is therefore derived from that previously described in that the nozzle of the micro-extruder is directed against the wall of the roller, which is a means of conveyance, and undergoes a periodic movement parallel to the said wall.
The invention also concerns the equipment designed to carry out the process of obtaining a layer, for example the inside of the tread, provided with a striation of periodic path, as described above. The said equipment comprises a normal extruder, also known as a flat die extruder, or a roller die extruder, for the extrusion of the said layer in the unvulcanized condition, and a micro-extruder fitted permanently on a support which also enables the said micro-extruder to be displaced along the three main directions, the said micro-extruder comprising mainly an extrusion head fined at its end with a nozzle designed to extrude the circumferential insert, with the desired section and path, into the section of unvulcanized mix emerging from the main extruder. The end of the said nozzle is directed against the roller of the main extruder or against the wall of the means of conveyance of the treated portion of the tread. More particularly, the invention concerns the nozzle or extrusion orifice of the micro-extruder used. The said nozzle is characterized in that it comprises a contact surface for attachment to the outlet of the micro-extruder by means of the attachment elements required for fixing it, the said surface extending axially to a nozzle head incorporating a hollow feed chamber having, upstream, a sharp cutting edge and provided, downstream, with an extrusion orifice with the desired cross-section through which the conductive rubber material is extruded.
The characteristics of the present invention will be better understood from the description below, which refers to the attached drawing.